The goal of these investigators continues to be the definition of the mechanism of sudden death, delineation of predisposing factors and developing methodologic approaches for identifying subjects at risk. An underlying hypothesis is that electrical instability of the myocardium antedates and predisposes to ventricular fibrillation (VF). The work will consist of five areas of study: 1) Cardiac Vulnerability - to determine properties of the ventricular vulnerable period in the normal and ischemic heart; 2) Electrical Instability - to study the underlying mechanism of R/T pulsing evoked VF, the duration of the susceptibility of VF following acute coronary artery occlusion and the change in response to pulsing as a function of size and location of infarction; 3) Protective Zone - to define the role of the conducting system in determining the properties of this zone; 4) Nonsustained VF - to expose the factors which favor spontaneous reversion and to attempt to induce nonsustained VF in the large mammalian heart; 5) Antiarrhythmic Measures - to elaborate methods suitable for the expeditious assessment of antiarrhythmic efficacy of drugs.